I've got 4500 miles on my TKCs; no cupping and still a fair amount of tread on the rear.

A new front tire is cheap, why in the world would you take ANY risk when the consequences worse case could be catastrophic and best case leaves you stranded? Those arrows are there for a reason even if it's a risk management arrow.

If you're tire is 50% worn you've used say $70 worth of the tire and have another potential $70 value left in it, so for that remaining $70 your risking your life and/or someone else's.